The hangover feeling is your body telling you that you are dehydrated, The best thing is (as SMTT said) drink loads of water before the hangover!!!!
But it still works during the hangover - its just not so pleasant.
Also i am a firm believer in the spliff method, and also the "drink more" theory, but these are not always as easily said than done.
but a shitload of water should do the trick!